This invention relates to limit switches used for detection of objects or the like such as safety limit switches which have passed certain safety standard tests including the dielectric strength test, the electrical endurance test and the cable strength test.
FIG. 29A shows an example of prior art limit switch S-1, comprising a switch unit 190 and a head unit 191. The switch unit 190 includes a case 192 and a plunger 193 and also contains an inner switch 194 with a mobile switch plunger 195. The switch plunger 195 supports two mobile members (not shown) and is contained inside a switch plunger holder (not shown) by means of a returning spring (not shown). Mobile contact points (not shown) on the mobile members and fixed points of a fixed terminal (not shown) form a contact point part. The head unit 191 includes a rotary shaft 197 rotatably supported by a head case 196. A roller lever 199 with a roller 198 is attached to an end part of this rotary shaft 197, and a cam 200 is provided to the rotary shaft 197. The limit switch is formed by attaching the head unit 191 to the switch unit 190 such that the plunger 193 is pushed against the cam 200 of the rotary shaft 197. The head unit 191 is attached to the switch unit 190 by placing the head unit 191 on top of the switch unit 190, pushing in screws (not shown) into holes formed in the head case 196 of the head unit 191 and engaging these screws in threaded holes (not shown) formed in the case 192 of the switch unit 190. This method of attachment, however, did not provide enough rigidity.
The limit switch is operated by moving a so-called dog (not shown) to cause the roller 198 of the limit switch to contact the dog and to tilt the roller lever 199. As the roller lever 199 is tilted, the rotary shaft 197 is rotated to push the plunger 193 such that the switch plunger 195 of the inner switch 194 is pressed and the inner switch carries out a switching operation.
With a prior art limit switch, if the number of circuits for the inner switch 194 is increased, the total number of components increases and the inner switch 194 becomes too large, causing the limit switch as a whole to become also too large. If each constituent parts is made small so as to prevent the limit switch from becoming too large, however, it becomes difficult to produce a switch which can pass safety tests.
For attaching the limit switch to a target structure 201 (such as a wall), as shown in FIG. 29B, a screw 202 is passed through a screw hole 192A in the case 192 of the switch unit 190 and is screwed into the target structure 201. With a prior art limit switch, the attachment between the head unit 191 to the switch unit 190 was not sufficiently rigid, as explained above. If an object 203 is unexpectedly inserted or falls into the space between the head unit 191 and the target structure 201 to which the limit switch is to be attached, as shown in FIG. 29B, the head unit 191 may tilt forward as the screw 202 is tightened, causing the limit switch be deformed or otherwise damaged. If the limit switch is provided with an attachment member for attaching it to a target structure (such as shown at 201) and if there is a gap provided between the attachment member and the target structure, furthermore, it is necessary to fill the gap with a spacer or the like for preventing deformation and breakage.
Another problem with prior art limit switches was that any attempt to improve their sensitivity in response to the market demand had adverse effects on resistance against vibrations, shocks and telegraphing.
It is therefore an object of this invention to provide a limit switch which need not be increased in size to satisfy the safety requirements of regulations although the total number of circuits is increased.
It is another object of the invention to provide a limit switch which is sufficiently strong and hence does not become deformed or damaged at the time of attachment and hence does not require extra components for preventing deformation and breakage or damage easily even if an unwanted object is inserted between the head unit and the target structure on which it is to be attached.
A limit switch embodying this invention may be characterized not only as comprising an inner switch having a plurality (say, more than four) of switch element units and an inner case, the inner case containing the inner switch and having covers and a switch case, a partition wall being provided such that independently provided switch areas sandwich the partition wall in between, and the switch element units being distributed individually into these switch areas, and the covers each sealing a corresponding one of the switch areas such that insulation characteristics can be improved. Screening members may be further provided for separating the switch element units when they are contained in the switch areas.
Each switch element unit comprises a fixed terminal having two (first and second) fixed contact points, a mobile member having two (first and second) mobile contact points and a switch plunger. The first fixed contact point and the first mobile contact point together form a first contact unit, and the second fixed contact point and the second mobile contact point together form a second contact unit. The switch plunger serves to open and close these contact units.
The switch areas include a part for containing the switch plunger (xe2x80x9cswitch plunger containing partxe2x80x9d), parts for attaching the fixed terminals (xe2x80x9cfixed terminal attachment partsxe2x80x9d) sandwiching the switch plunger containing part from both sides, and lead line areas for keeping lead lines disposed along the fixed terminal installation parts and connected to the fixed terminals. With the switch areas thus structured, the number of switch element units may be increased without increasing the size of the limit switch without adversely affecting the safety characteristics.
The fixed terminal attachment parts include a plurality of fixed terminal installation parts for detachably installing the fixed terminals along the direction in which the switch plunger moves. The switch plunger comprises a switch plunger main body having a plurality of mobile members detachably attached and an elastic holder for holding and applying a biasing force on the mobile members. The fixed terminals are detachably set to the fixed terminal installation parts, and the switch plunger is biased by a returning spring when contained movably inside the switch plunger containing part. With a limit switch thus structured, the structure of contact units can be modified without using different components and only by changing the position of the mobile member, making it possible to develop an increased number of variations. Moreover, since common components will be used, it does not incur any extra cost to develop such variations in the structure of the contact units.
A limit switch of this invention may comprise plunger means for operating inner switches, including a link plunger for operating switch plungers of a plurality of inner switches. With such a structure, a link plunger as a different component can operate the switch plunger s of a plurality of inner switches and since this does not depend on the shape of the actuator, it becomes easier to develop variations.
The main body of the case may be provided with a hole part for the link plunger to pass through and a seal cap holder at this hole part for holding a seal cap. The link plunger has a plunger engaging part. The seal cap comprises a cap main body made of an elastic material with two (first and second) engaging parts at end parts of the cap main body, one of them formed as an O-ring. The link plunger is attached to the seal cap by engaging the first engaging part with the plunger engaging part and the second engaging part with the seal cap holder. Since the seal cap has the function of an O-ring, a more reliable sealing can be effected and the link plunger can follow the motion of the switch plunger more efficiently.
The inner case may also include a fastening case such that the cable of lead lines will be sandwiched between the switch case and this fastening case as the lead lines of the cable are connected to the inner switch and the switch case is fastened to the fastening case. With such a structure, the sealing resin (adhesive) can be effectively prevented from flowing into the inner switch.
The fastening case may be connected to the lead lines of the cable. The cable may have a sealing member with a sealing ring outside. The fastening case may have a cable-passing hole for passing the cable through. The sealing member is inserted into this cable-passing hole. The fastening case has seal holders and the cable is sealed by pressing the sealing ring with the seal holders.
One advantage of connecting the lead lines to the fixed terminal by compression is that solder does not have to be used for the purpose and the production method is gentler to the environment.
Compressing means such as protrusions may be provided on the inner surface of the switch cover for a portion of the lead lines. When a plurality of switch element units are disposed separately in the switch areas, each being covered by its own switch cover, the connection of the lead lines can be secured because such compression means is used to apply compressive force.
The switch case may be provided with an elastic grounding line holder for holding a grounding line and causing it to contact the main body of the switch. One advantage of this method of attachment is that it is not necessary to drill a hole through a portion of the outer case where the grounding line is to be attached.
The invention also teaches to arrange the lead lines symmetrically inside the cable such that the limit switch can be made compact and the arrangement of the lead lines becomes simpler.
In order to improve the rigidity, the limit switch of this invention may be characterized as comprising a switch unit, a head unit which operates the switch unit, a protrusion on either of the switch unit and the head unit and an indentation on the other such that the head unit is connected to the switch unit by engaging the protrusion in the indentation. Rigidity improves because the protrusion and the indentation engage each other in a manner of embracing each other. The switch unit comprises a switch main body having shoulders, the protrusion being formed by steps at the shoulders. The head unit has a head case with two legs protruding to form the indentation therebetween. The protrusion has an attachment hole part. A screw member is inserted in this attachment hole part and compresses the legs to the protrusion so as to fasten the switch main body and the head case together. The switch unit contains an inner switch, and the limit switch further comprises a head unit displacing means for moving the head unit in the direction of movement of the switch plunger. The head unit displacing means may include a spacer inserted between the head unit and the switch unit.
The head unit may include a head case and a rotary shaft rotatably supported by the head case, the head case displays two or more set zones, the rotary shaft having a zone indicator for selectively indicating one of these set zones. When two or more inner switches with different operating characteristics are included, these set zones can be selectively utilized. Since only one head unit is needed for two or more inner switches, the present invention contributes to the reduction in cost.
The limit switch may comprise a lever for rotating the rotary shaft, a lever angle adjuster supporting the lever, and a roulette part formed on a portion of a peripheral part of the lever angle adjuster. The rotary shaft and the lever angle adjuster engage to each other through the roulette part. This lever may be a form lock lever having a pair of holes which are elongated in the direction of the lever and gear parts on the elongated holes. The lever angle adjuster is provided with a pair of lever holders with lever engaging parts. The lever holders are inserted to the elongated holes so as to engage the gear parts with the lever engaging parts so as to connect the form lock lever with the lever angle adjuster.
The head unit includes an actuator for operating the switch head. The actuator comprises a roller to be contacted by a mobile member and a lever which supports the roller. The lever tilts when the mobile member contacts the roller. A weight may be provided on the lever at a position opposite to the roller.